1. Technical Field
This invention relates generally to heat treat fixtures and methods for supporting thin sheet metal work pieces during heat treatment in liquid heating and cooling media to achieve hardening of the work pieces.
2. Related Art
The heat treatment of thin sheet-like structures such as metal gasket layers and the like is generally avoided in favor of the usage of pre-hardened materials such as 301 full hard stainless steel (FHSS) in the case of metal gasket layers. 301 FHSS starting material is roll hardened and possesses the desired end hardness and strength needed for metal gasket applications, which typically include one or more active layers formed with bead embossments that project out of the plane of the gasket layer and serve when compressed to provide a resilient seal between adjoining clamped structures such as a head and block of an engine. The bead embossments are typically stamped in an initially flat sheet of 301 FHSS via a stamping operation and, as such, the selection of material for the active layers must be sufficiently ductile to allow for such formation of the beads, yet sufficiently hard and strong in use to withstand considerable loading and deformation without cracking or yielding plastically under load.
It is generally accepted that the approach of heat treat hardening such thin sheet-like work pieces fabricated of less expensive heat treat hardenable materials, while attractive from a cost standpoint, is impractical at best, since such sheet-like structures having a considerably large surface area together with an extremely thin cross section (for metal gasket layers, typically on the order of about 0.01 inches) and are, by their nature, inherently unstable in a heat treat environment and would have a tendency to warp beyond levels acceptable in metal gasket applications when exposed to the extreme and rapid changes in temperature required to achieve heat treat hardening of the material.
Known approaches to controlling the warpage have involved constraining the thin sheets between two plates to prevent all movement of the sheet both against movement within its plane and out of its plane. Such approaches are not known to have been successful at preventing warpage and would likely worsen the condition by setting up non-uniform heating and cooling rates across the surface of the work piece which would contribute further to resultant warpage.
U.S. Pat. No. 5,310,196 discloses provision of a heat treat gasket layer, but the disclosure is silent as to the particulars of the heat treat process, including any fixturing of the parts, to achieve hardening while preserving the dimensional stability of the work pieces.
Accordingly, there is a need in the industry for a heat treat process and fixture apparatus capable of achieving effective, practical heat treat hardening of thin sheet work pieces such as metal gasket layers while preventing warpage of such work pieces that is prevalent using known conventional heat treatment and fixturing techniques.